(1) Field of the Invention
This invention relates to a method of separating a mixed liquid by rectification into light and heavy fractions with a difference in boiling point therebetween of at least 15.degree. C.
(2) Description of the Prior Art
Dimethylformamide (hereinafter referred to simply as DMF) which is widely used in the production of synthetic fibers and resins, and in that case it is generally recovered in the form of a 10-20 wt % aqueous solution. In the Kraft pulp industry, the waste liquor which is discharged from a cooker and evaporator process contains about 0.5 wt % of methanol and other volatile substances. It is the general practice to resort to the distillation in the recovery of respective components from such a dilute liquid. However, since the object component or components exist in the liquid only in small concentrations, a large consumption of thermal energy is compelled in order to recover the desired component or components in a high concentration.
Meanwhile, there has been known in the art the so-called vapor recompression type distillation process for fractionating a liquid which consists of heavy and light components, wherein the vapor from the top of the tower, which is rich in the light component, is compressed by a compressor to a temperature level 5.degree. to 10.degree. C. higher than that of the bottom liquid, which is rich in the heavy component, the compressed vapor being fed to a reboiler for heating the bottom liquid by transferring the heat of condensation of the vapor to the bottom liquid, thereby to recover the entire quantity or a major proportion of the heat which is required for the distillation. This method, however, can be advantageously applicable only in a case where the difference in boiling point between the top and bottom fractions is small, because the power consumption by the compressor becomes larger with the increases in the P.sub.d /P.sub.s ratio (a ratio of the pressure P.sub.d on the discarge side of the compressor to the pressure P.sub.s on its suction side). In this instance, the value P.sub.s is almost the same as the pressure at the tower top while the value P.sub.d is the pressure of the vapor of the tower top at a temperature which is the sum of the temperature of the bottom liquid plus the effective temperature difference at the heat transfer surface of the reboiler, so that the ratio P.sub.d /P.sub.s is remarkably incresed with the increase in temperature difference between the tower top and the tower bottom. Thus, the vapor recompression type distillation, in which the heat required for the distillation results from the driving power of the compressor rather than from the thermal energy supplied from outside of the system, is generally applied advantageously only for the separation of the light fraction from the heavy fraction in which the difference in boiling point therebetween in smaller than 15.degree. C. Accordingly, no attempt has ever been made to apply it to such a liquid which consists of components with large differences in boiling points like the abovementioned DMF-water or methanol-water system.